1. 1 Integrity Policies CSSE 490 Computer Security Mark Ardis, Rose-Hulman Institute March 22, 2004

2. 2 Acknowledgements Many of these slides came from Matt Bishop, author of Computer Security: Art and Science

3. 3 Overview Requirements Very different than confidentiality policies Biba’s models Low-Water-Mark policy Ring policy Lipner’s model Combines Bell-LaPadula, Biba Clark-Wilson model

4. 4 Requirements of Policies 1. Users will not write their own programs, but will use existing production programs and databases. 2. Programmers will develop and test programs on a nonproduction system; if they need access to actual data, they will be given production data via a special process, but will use it on their development system. 3. A special process must be followed to install a program from the development system onto the production system. 4. The special process in requirement 3 must be controlled and audited. 5. The managers and auditors must have access to both the system state and the system logs that are generated.

5. 5 Biba Integrity Model Basis for all 3 models: Set of subjects S, objects O, integrity levels I, relation ≤  I  I holding when second dominates first min: I  I  I returns lesser of integrity levels i: S  O  I gives integrity level of entity r: S  O means s  S can read o  O w, x defined similarly

6. 6 Intuition for Integrity Levels The higher the level, the more confidence That a program will execute correctly That data is accurate and/or reliable Note relationship between integrity and trustworthiness Important point: integrity levels are not security levels

7. 7 Low-Water-Mark Policy Idea: when s reads o, i(s) = min(i(s),i(o)); s can only write objects at lower levels Rules 1. s  S can write to o  O if and only if i(o) ≤ i(s). 2. If s  S reads o  O, then i´(s) = min(i(s), i(o)), where i´(s) is the subject’s integrity level after the read. 3. s 1  S can execute s 2  S if and only if i(s 2 ) ≤ i(s 1 ).

8. 8 Problems Subjects’ integrity levels decrease as system runs Soon no subject will be able to access objects at high integrity levels Alternative: change object levels rather than subject levels Soon all objects will be at the lowest integrity level Crux of problem is model prevents indirect modification Because subject levels lowered when subject reads from low-integrity object

9. 9 Ring Policy Idea: subject integrity levels static Rules 1. s  S can write to o  O if and only if i(o) ≤ i(s). 2. Any subject can read any object. 3. s 1  S can execute s 2  S if and only if i(s 2 ) ≤ i(s 1 ). Eliminates indirect modification problem

10. 10 Lipner's Integrity Matrix Model Steve Lipner proposed this as first realistic commercial model Combines Bell-LaPadula, Biba models to obtain model conforming to requirements Do it in two steps Bell-LaPadula component first Add in Biba component

11. 11 Bell-LaPadula Clearances 2 security clearances/classifications AM (Audit Manager): system audit, management functions SL (System Low): any process can read at this level

12. 12 Bell-LaPadula Categories 5 categories D (Development): production programs in development but not yet in use PC (Production Code): production processes, programs PD (Production Data): data covered by integrity policy SD (System Development): system programs in development but not yet in use T (Software Tools): programs on production system not related to protected data

13. 13 Users and Security Levels SubjectsSecurity Level Ordinary users(SL, { PC, PD }) Application developers(SL, { D, T }) System programmers(SL, { SD, T }) System managers and auditors (AM, { D, PC, PD, SD, T }) System controllers(SL, {D, PC, PD, SD, T}) and downgrade privilege

14. 14 Objects and Classifications ObjectsSecurity Level Development code/test data(SL, { D, T }) Production code(SL, { PC }) Production data(SL, { PC, PD }) Software tools(SL, { T }) System programs (SL,  ) System programs in modification (SL, { SD, T }) System and application logs(AM, { appropriate })

15. 15 Ideas Ordinary users can execute (read) production code but cannot alter it Ordinary users can alter and read production data System managers need access to all logs but cannot change levels of objects System controllers need to install code (hence downgrade capability) Logs are append only, so must dominate subjects writing them

16. 16 Check Requirements 1. Users have no access to T, so cannot write their own programs 2. Applications programmers have no access to PD, so cannot access production data; if needed, it must be put into D, requiring the system controller to intervene 3. Installing a program requires downgrade procedure (from D to PC), so only system controllers can do it

17. 17 More Requirements 4.Control: only system controllers can downgrade; audit: any such downgrading must be audited 5.System management and audit users are in AM and so have access to system state and logs

18. 18 Clark-Wilson Integrity Model Integrity defined by a set of constraints Data in a consistent or valid state when it satisfies these Example: Bank D today’s deposits, W withdrawals, YB yesterday’s balance, TB today’s balance Integrity constraint: D + YB – W = TB Well-formed transaction move system from one consistent state to another Issue: who examines, certifies transactions done correctly?

19. 19 Entities CDIs: constrained data items Data subject to integrity controls UDIs: unconstrained data items Data not subject to integrity controls IVPs: integrity verification procedures Procedures that test the CDIs conform to the integrity constraints TPs: transaction procedures Procedures that take the system from one valid state to another

20. 20 Certification Rules 1 and 2 CR1When any IVP is run, it must ensure all CDIs are in a valid state CR2For some associated set of CDIs, a TP must transform those CDIs in a valid state into a (possibly different) valid state Defines relation certified that associates a set of CDIs with a particular TP Example: TP balance, CDIs accounts, in bank example

21. 21 Enforcement Rules 1 and 2 ER1The system must maintain the certified relations and must ensure that only TPs certified to run on a CDI manipulate that CDI. ER2The system must associate a user with each TP and set of CDIs. The TP may access those CDIs on behalf of the associated user. The TP cannot access that CDI on behalf of a user not associated with that TP and CDI. System must maintain, enforce certified relation System must also restrict access based on user ID (allowed relation)

22. 22 Users and Rules CR3The allowed relations must meet the requirements imposed by the principle of separation of duty. ER3The system must authenticate each user attempting to execute a TP Type of authentication undefined, and depends on the instantiation Authentication not required before use of the system, but is required before manipulation of CDIs (requires using TPs)

23. 23 Logging CR4All TPs must append enough information to reconstruct the operation to an append-only CDI. This CDI is the log Auditor needs to be able to determine what happened during reviews of transactions

24. 24 Handling Untrusted Input CR5Any TP that takes as input a UDI may perform only valid transformations, or no transformations, for all possible values of the UDI. The transformation either rejects the UDI or transforms it into a CDI. In bank, numbers entered at keyboard are UDIs, so cannot be input to TPs. TPs must validate numbers (to make them a CDI) before using them; if validation fails, TP rejects UDI

25. 25 Separation of Duty In Model ER4Only the certifier of a TP may change the list of entities associated with that TP. No certifier of a TP, or of an entity associated with that TP, may ever have execute permission with respect to that entity. Enforces separation of duty with respect to certified and allowed relations

26. 26 Comparison With Requirements 1. Users can’t certify TPs, so CR5 and ER4 enforce this 2. Procedural, so model doesn’t directly cover it; but special process corresponds to using TP No technical controls can prevent programmer from developing program on production system; usual control is to delete software tools 3. TP does the installation, trusted personnel do certification

27. 27 Comparison With Requirements 4. CR4 provides logging; ER3 authenticates trusted personnel doing installation; CR5, ER4 controll installation procedure New program UDI before certification, CDI (and TP) after 5. Log is CDI, so appropriate TP can provide managers, auditors access Access to state handled similarly

28. 28 Comparison to Biba Biba No notion of certification rules; trusted subjects ensure actions obey rules Untrusted data examined before being made trusted Clark-Wilson Explicit requirements that actions must meet Trusted entity must certify method to upgrade untrusted data (and not certify the data itself)

29. 29 Key Points Integrity policies deal with trust As trust is hard to quantify, these policies are hard to evaluate completely Look for assumptions and trusted users to find possible weak points in their implementation Biba, Lipner based on multilevel integrity Clark-Wilson focuses on separation of duty and transactions